RADIOGRAPHICS FUNDAMENTALS | MUSCULOSKELETAL IMAGING 2038 Musculoskeletal MRI Pulse Sequences: A Review for Residents and Fellows Stephanie Jo, MD, PhD Steffen Sammet, MD, PhD Stephen Thomas, MD G. Scott Stacy, MD Abbreviations: CHESS = chemical shift–selec- tive saturation, GRE = gradient echo, SE = spin echo RadioGraphics 2019; 39:2038–2039 http://doi.org/10.1148/rg.2019190075 Content Codes: From the Department of Radiology, University of Pennsylvania, Philadelphia, Pa (S.J.); and De- partment of Radiology, University of Chicago, Chicago, Ill (S.S., S.T., G.S.S.). Presented as an education exhibit at the 2018 RSNA Annual Meeting. Received March 29, 2019; revision re- quested May 10 and received June 1; accepted June 18. All authors have disclosed no relevant relationships. Address correspondence to S.J., Penn University City, 3737 Market St, Mailbox 4, Philadelphia, PA 19104 (e-mail: stephanie.jo@ pennmedicine.upenn.edu). © RSNA, 2019 The full digital presentation is available online. MRI is widely used for the evaluation of musculoskeletal conditions. There has been continual improvement in the acquisition meth- ods and commonly used pulse sequences in musculoskeletal MRI. Understanding the physics behind image creation and the rationale behind sequence selection is important for proper interpretation of MR images, as well as for improving future imaging protocols as new techniques become available. In MRI, proton spins in the human body generate a magnetic moment in an external magnetic field (B 0 ). The external magnetic field of clinical MR magnets typically ranges from 0.3 T to 7 T and can generate a magnetization vector that can be used for perform- ing MRI. A radiofrequency field (B 1 ) flips the magnetization vector in the transverse plane. After the radiofrequency field is turned off, the magnetization will undergo relaxation processes with the time constants T1, T2, and T2*, which vary for different types of tissue. Using these relaxation time differences, two major MR pulse se- quence types have been developed to create MR images: spin-echo (SE) and gradient-echo (GRE) sequences. SE sequences use a 90° degree excitation pulse followed by a 180° refocusing pulse, whereas GRE sequences generally use a single radiofrequency pulse followed by dephasing and rephasing gradients. GRE sequences allow shorter imaging times but are more susceptible to image artifacts. Fat suppression is critical for the detection of pathologic conditions at MRI. Chemical shift–selective saturation (CHESS) and inversion recovery are commonly used techniques for fat suppression in muscu- loskeletal imaging. The CHESS technique uses the microenvironment differences of protons in water and fat, whereas the inversion-recovery technique uses T1 relaxation time differences between water and fat protons. Understanding the different fat-suppression techniques can aid in lesion characterization and sequence selection in the presence of metallic hardware and gadolinium-based contrast material (Figure). Several artifacts are common and relevant to musculoskeletal imaging. These include motion, susceptibility, and magic angle TEACHING POINTS ■ SE pulse sequences are fundamental sequences used for most musculoskeletal MRI proto- cols. GRE pulse sequences can offer more rapid acquisition and different contrast proper- ties but are prone to imaging artifacts, which can be helpful or detrimental to diagnosis. ■ Fat suppression is critical for the detection of pathologic conditions at MRI. CHESS and inversion-recovery techniques are commonly used for fat suppression in musculoskeletal imaging. CHESS techniques are more prone to magnetic field inhomogeneities but can be used with gadolinium-based contrast material. Inversion recovery–based techniques are less susceptible to magnetic field inhomogeneities but may decrease the signal inten- sity of contrast material. ■ Motion, magnetic susceptibility, and magic angle artifacts are common in musculoskel- etal MRI, and recognizing these artifacts is important to avoid misdiagnosis. This copy is for personal use only. To order printed copies, contact reprints@rsna.org